Mathis, 33, is out for the season after tearing his Achilles earlier this month during a personal workout. He was serving a four-game suspension for violating the league's performance-enhancing drug policy for taking what later was revealed to be a fertility drug.
Albert Breer of the NFL Network reports Mathis will still be paid his $6 million salary this season while he is on the non-football injury list, and the extension will also pay him $6 million for the 2016 season. Mathis signed a four-year, $36 million contract with the Colts in 2012.
Mathis, a five-time Pro Bowl selection, led the NFL with 19.5 sacks last season and had 111 sacks during his first 12 seasons with the Colts. Without Mathis, the Colts have dropped from 13th in yards allowed last season to 19th this year. The Colts' 42.0 sacks ranked 11th in the league last season, and they are tied for 13th this season with 8.0 sacks after four games.
The Colts started the season 0-2 after allowing 30.5 points per game against the Denver Broncos and Philadelphia Eagles. They have rebounded to win their last two games against the Jacksonville Jaguars and Tennessee Titans by limiting them to 17 points per game.
Indianapolis hosts the Baltimore Ravens on Sunday at 1 p.m.
- Paul Palladino